1. Field of the Invention
This invention relates to a process for regenerating unreacted vinyl chloride monomers recovered from a process of vinyl chloride polymer production, which has a characteristic feature in the step of compressing the monomer.
2. Description of the Prior Art
Vinyl chloride polymers (inclusive of vinyl chloride type polymers and copolymers of vinyl chloride with other copolymerizable monomers) are conventionally produced by subjecting, in a polymerization vessel, vinyl chloride monomer or a mixture of vinyl chloride monomer and other copolymerizable monomer to polymerization reaction in the presence of a polymerization initiator by a process such as suspension polymerization or emulsion polymerization. After the polymerization is completed, a gaseous unreacted vinyl chloride monomer is recovered from the interior of the polymerization vessel (when the above monomer mixture is used, the unreacted vinyl chloride monomer is recovered in a special way), and then the vinyl chloride polymer is drawn out of the interior of the polymerization vessel. The vinyl chloride polymer drawn out of the interior of the polymerization vessel is made into products through the steps of dehydration and drying. Meanwhile, the gaseous unreacted vinyl chloride monomer recovered from the interior of the polymerization vessel (hereinafter often xe2x80x9crecovered VCM or xe2x80x9crecovered VCM gasxe2x80x9d) is rectified, and reused as a material for polymerization, vinyl chloride monomer.
Usually, the VCM gas recovered from the interior of the polymerization vessel is sent to a compressor and is compressed into a high-pressure gas, which is thereafter sent to a condenser, where it is liquefied into a crude liquefied recovered VCM. This crude liquefied recovered VCM is further sent to a hydro-extracting tank to separate water, and thereafter rectified in a rectifier. The product obtained is stored as a recovered VCM. The recovered VCM thus stored is thereafter reused as a material for polymerization, liquefied VCM.
In the step of compressing the recovered VCM, the compression of the recovered VCM fed brings about a rise of temperature of the recovered VCM and the resultant compressed recovered VCM to tend to cause polymerization. Accordingly, a method is employed in which the compressor is cooled and oil-lubricated (meant to be lubricated with a lubricating oil) at its driving portion and rubbing portions, and also, in order to keep the temperature of recovered VCM from rising, a lubricating oil is put and fed into the compressor in the step of compressing the recovered VCM, so as to be brought into contact with the recovered VCM to cool the recovered VCM and others.
According to this method, what is discharged out of the compressor is a mixture of the compressed recovered VCM gas and the lubricating oil. Accordingly, since the mixture discharged out of the compressor must be separated into the compressed recovered VCM gas and the lubricating oil, the mixture is sent to a gas-liquid separator and is separated there into the compressed recovered VCM gas and the lubricating oil. The compressed recovered VCM thus separated is sent to the condenser, and is finally reused as a material for polymerization, liquefied VCM, as stated previously.
Meanwhile, the lubricating oil separated is sent to a lubricating-oil tank, and is put into the compressor in the above compression step so as to be reused as the lubricating oil to be fed.
However, where the lubricating oil separated is fed into the compressor in the step of compressing the recovered VCM and circulated and reused for a long period of time as a circulating lubricating oil, the following difficulties may occur.
1) The vinyl chloride polymer forms on and adheres to the inner walls of piping which forms a circulation line of the lubricating oil or to the interior of a strainer provided in the course of the piping. In an extreme case, the piping clogs because of such adhesion of the vinyl chloride polymer.
2) The vinyl chloride polymer forms on and adheres to the inner walls of a lubricating-oil cooler provided in the course of the piping of the lubricating-oil circulation line, to cause a decrease in overall heat transfer coefficient (U-value) to lower the effect of cooling the lubricating oil circulating therethrough. Hence, there has been a problem that the lubricating-oil circulation line must frequently be disassembled and cleaned.
Accordingly, an object of the present invention is to provide a process for regenerating unreacted vinyl chloride monomers which process may hardly cause the polymerization of vinyl chloride in the lubricating-oil circulation line and so forth even when the lubricating oil fed into the compressor in the step of compressing the unreacted vinyl chloride monomer recovered is circulated and reused as a circulating lubricating oil over a long period of time.
The present inventors made extensive studies on the above problems the prior art has had. As the result, they have discovered that, even after the lubricating oil fed into the compressor and used therein has been separated from the recovered VCM by the gas-liquid separator and recovered, a recovered VCM involving a residue of a polymerization initiator stands dissolved therein in a very small quantity, and hence, when this lubricating oil is circulated to the compressor, the polymerization more tends to take place therein because of a high temperature accompanying the compression of the recovered VCM, so that, where the lubricating oil is circulated over a long period of time in such a state to continue its reuse, the vinyl chloride polymer is formed to cause the clogging or blocking of the strainer and the decrease in overall heat transfer coefficient of the cooler. Accordingly, the present inventors incorporated a polymerization inhibitor in the circulating lubricating oil, whereby the polymerization of vinyl chloride was effectively kept from occurring and the above difficulties were effectively restrainable even when the circulating lubricating oil is reused over a long period of time. On the basis of this finding, they have accomplished the present invention.
That is, the present invention provides a process for regenerating unreacted vinyl chloride monomers; the process having the steps of:
compressing in an compressor an unreacted vinyl chloride monomer recovered from a process of vinyl chloride polymer production, in contact with a lubricating oil fed into the compressor; and
separating a mixture of a compressed unreacted vinyl chloride monomer and the lubricating oil, thus formed, into the compressed unreacted vinyl chloride monomer and the lubricating oil;
wherein the lubricating oil contains a polymerization inhibitor.
The lubricating oil thus separated from the mixture of the compressed unreacted vinyl chloride monomer and the lubricating oil can be circulated to the compressor so as to be reused.
The compressed unreacted vinyl chloride monomer separated by the above process from the mixture of the compressed unreacted vinyl chloride monomer and the lubricating oil may be purified so that it can be reused or can be stored as a liquefied vinyl chloride monomer so as to be reused. The purification of the compressed unreacted vinyl chloride monomer is performed as follows, for example. The compressed recovered vinyl chloride monomer separated from the mixture is sent to a condenser, where it is liquefied into a crude liquefied vinyl chloride monomer. This crude liquefied recovered vinyl chloride is further sent to a hydro-extracting tank to separate water, and thereafter rectified in a rectifier. The vinyl chloride monomer thus rectified is stored in a liquefied state in a tank, and thereafter reused as a starting material for polymerization.
According to the present invention, the lubricating oil fed into the compressor in the step of compressing the unreacted vinyl chloride monomer contains a polymerization inhibitor, and hence the polymerization of vinyl chloride may hardly cause the polymerization of vinyl chloride in the lubricating-oil circulation line and others even when the lubricating oil is circulated and reused as a circulating lubricating oil over a long period of time. As the result, the clogging or blocking of the strainer, caused by the vinyl chloride polymer, and the decrease in overall heat transfer coefficient of the cooler can effectively be kept from occurring. Hence, the lubricating-oil circulation lines may be disassembled and cleaned much less times than ever.